Hi Nyn,
I wrote it this way, as a first attempt, so that no-one would end up with a file maintenance problem. I thought some less technical users may not want to have to delete old covers manually in a file browser. I'll have a think whether I can offer both options:

copy & replace all existing covers (as now)

copy & add to existing covers

whilst still retaining a measure of automatic file maintenance.

In the meantime you can simulate adding to existing covers by including the book covers you want to keep with the new book covers when you do the next Copy Cover from the calibre library window.

I'd also like to add covers without losing existing ones. it's not easy to resend old, with all without copious use of custom columns in calibre to get then all showing at the same time same main browse window ready to send

as most readers are designed to hold 100s of books, if not 1000s, and as a cover picture file is usually is no larger than a single book, I don't think you have to worry about there being too many covers sent to device

Two parameters in customise plug in would sort it.:
1. remove old cover(s) when sending new covers- choose: yes/no
2. set max number of covers allowed , enter a number or leave blank/use zero for unlimited.

if user opts for keep old and also for setting a max value then remove oldest files first, or just disallow that combination if it's too much trouble.